1. Field of the Invention
This invention relates to an improvement in a sintered diamond compact or high pressure form boron nitride (BN) compact and more particularly, it is concerned with a sintered diamond compact or high pressure form BN compact with an improved brazability, used for wear resisting tools, cutting tools, drill bits, dressers, wire-drawing dies and the like.
2. Description of the Prior Art
Diamond compacts obtained by sintering diamond fine grains with an iron group metal as a binder under an ultra-high pressure and high temperature have a more excellent wear resistance as edge materials of cutting tools, wire-drawing dies or drill bits, as compared with the prior art cemented carbides. In many cases, these compacts are bonded directly or through an interlayer to substrates of cemented carbides, as shown in FIG. 1 (a) and (b). The cemented carbides are used for the purpose of making it possible to braze a diamond compact to a tool holder or reinforcing a diamond compact.
In the case of a use wherein a diamond compact has a larger volume than its substrate as shown in FIG. 2, however, the bonding strength is often insufficient when only the substrate, i.e. cemented carbide is brazed. In this case, it has been considered to coat a diamond compact 1 with a cemented carbide 2 on the outer circumference and bottom of the diamond compact as shown in FIG. 3 (a) and (b) to increase the brazing area, but this results in a problem that the shape of a diamond compact is limited and it is hard to obtain a dimensional precision. When the volume of a diamond compact is larger than that of a cemented carbide, there arises a problem that cracks tend to occur in the diamond compact or cemented carbide due to the residual stress during sintering under an ultra-high pressure or the stress resulting from the differences of thermal expansion among the diamond compact, cemented carbide and a shank to be brazed.
Furthermore, these composite compacts of diamond or high pressure form BN meet with a disadvantage that the part of the diamond compact or high pressure form BN 1 is hardly wettable with a brazing filler 4 to form a clearance between the compact part 1 and a holder 3, as shown in FIG. 4 (a) and (b). When a stress in parallel with the bonded surface with a cemented carbide substrate 2 acts as shown by arrows in FIG. 4 (a) and (b), therefore, the effect of reinforcing by the cemented carbide substrate is lost, resulting in occurrence of cracks in the compact and breakage of the edge part.
Accordingly, the compacts of the prior art can be applied to only uses wherein a stress is loaded in the perpendicular direction to the bonded surface with a cemented carbide substrate. When these compacts are small-sized, there arises a problem that since only the cemented carbide substrate is brazed, the brazing strength is lowered and the brazing is get out of place.
In the specification the designation of groups IVa, Va and VIa of the periodic table corresponds to the conventional U.S. designation of groups IVb, Vb, and VIb in the periodic table.
A compact of high pressure form boron nitride bonded with a suitable binder has an excellent performance for cutting hardened steels, cast iron and heat resisting alloys, since high pressure form BN has less reactivity with iron group metals. In particular, a compact of cubic boron nitride (CBN), one kind of high pressure form boron nitrides, bonded with a carbide, carbonitride or nitride of Group IVa, Va or VIa element of Periodic Table, and an aluminum compound has an excellent wear resistance and toughness for cutting the above described workpieces. In many cases, these compacts are bonded directly or through an interlayer to substrates of cemented carbides, as shown in FIG. 1 (a) and (b). The cemented carbides are used for the purpose of making it possible to braze a CBN compact to a tool holder or reinforcing a CBN compact.
In the case of a use wherein a high pressure form BN has a larger volume than its substrate as shown in FIG. 2, however, the bonding strength is often insufficient when only the cemented carbide part is brazed. In this case, if a high pressure form BN compact is coated with a cemented carbide for brazing, there arises a problem that the shape of the high pressure form BN compact is limited. When the volume of a high pressure form BN compact is larger than that of a cemented carbide, furthermore, there arises a problem that cracks tend to occur in the BN compact or cemented carbide due to the residual stress during sintering under an ultra-high pressure or the stress resulting from the differences of thermal expansion among the BN compact, cemented carbide and a shank to be brazed.
At the present time, diamond compacts for tools have been marketed in which at least 70% by volume of diamond grains are bonded with each other. These compacts are used as cutting tools of non-ferrous metals, plastics or ceramics, dressers, drill bits or wire-drawing dies. When using the diamond compact for cutting non-ferrous metals or for drawing relatively soft wires such as copper wires, in particular, it exhibits very excellent properties.
These diamond compacts are ordinarily sintered by using, as a binder, an iron group metal such as cobalt, the catalyst for the synthesis of diamond. Therefore, the diamond compacts have a disadvantage that when heating at a temperature of higher than 600.degree. C., diamond is graphitized and degraded. Thus, in order to improve the heat resistance of the diamond compact, it has been proposed to remove the iron group metal such as cobalt promoting graphitization of diamond during heating, as disclosed in Japanese Patent Application OPI (Kokai) No. 114589/1978. The diamond compact from which the solvent metal has thus been removed is capable of surely resisting a temperature of up to about 1200.degree. C. in high vacuum.
However, the solvent metal-removed diamond compact has still an insufficient heat resistance when heated in the air. When the diamond compact being a porous body is heated at a temperature of up to 900.degree. C., diamond grains on and inside the surface are brought into direct contact with oxygen, thus resulting in graphitization and degradation of the surfaces of the diamond grains.
The diamond compact is fitted to a bit main body or shank, as an edge of a drill bit or dresser, by the use of a matrix or brazing alloy. The holding strength of a matrix is increased with the increase of the melting point of the matrix material, so in the case of using the above described diamond compact as an edge material of a drill bit for drilling hard rocks, in particular, it is necessary to use a brazing alloy and a matrix each having a melting point of higher than 900.degree. C. When fitting the diamond compact under the situation, therefore, the diamond compact is heated at a temperature of 800.degree. to 1100.degree. C. in the air and thus degraded, so that it is impossible to obtain a sufficient strength in the resulting drill bit or dresser.
Thus, it will be understood that a further improvement of the heat resistance of the diamond compact can be achieved by preventing it from exposure to the atmosphere during heating and based on this understanding, a heat resistance diamond compact has lately been marketed which is prepared, for example, by coating the surface of a diamond compact with nickel.